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            <h2 class="toc-title">目录</h2>
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        </div><article class="page single"><h1 class="single-title animate__animated animate__flipInX">Java并发编程之无锁</h1><div class="post-meta">
            <div class="post-meta-line"><span class="post-author"><a href="/" title="Author" rel="author" class="author"><i class="fas fa-user-circle fa-fw" aria-hidden="true"></i>MC</a></span>&nbsp;<span class="post-category">收录于 <a href="/categories/java/"><i class="far fa-folder fa-fw" aria-hidden="true"></i>Java</a></span></div>
            <div class="post-meta-line"><i class="far fa-calendar-alt fa-fw" aria-hidden="true"></i>&nbsp;<time datetime="2018-10-09">2018-10-09</time>&nbsp;<i class="fas fa-pencil-alt fa-fw" aria-hidden="true"></i>&nbsp;约 2509 字&nbsp;
                <i class="far fa-clock fa-fw" aria-hidden="true"></i>&nbsp;预计阅读 6 分钟&nbsp;<span id="/java%E5%B9%B6%E5%8F%91%E4%B9%8B%E6%97%A0%E9%94%81/" class="leancloud_visitors" data-flag-title="Java并发编程之无锁">
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  <ul>
    <li><a href="#无锁解决冲突的办法cas">无锁解决冲突的办法：CAS</a></li>
    <li><a href="#java中的指针unsafe类">Java中的指针：Unsafe类</a></li>
    <li><a href="#atomicreference-无锁的引用">AtomicReference 无锁的引用</a></li>
    <li><a href="#atomicstampedreference">AtomicStampedReference</a></li>
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            </div><div class="content" id="content"><h1 id="java并发编程之无锁">Java并发编程之无锁</h1>
<p>在谈论无所之前先来看看乐观派和悲观派。对于乐观派而言，他们总认为事情总会朝着好的方向发展，总认为几乎不会坏事，我已可以随意的去做事。但是对于悲观派来说，他们认为出错是一种常态，所以无论事情大小都会考虑的面面俱到，滴水不漏。</p>
<p>在两种派别对应在并发中就是加锁和无锁，也就是说加锁是一种悲观的策略，而无锁是一种乐观的策略。对于锁，如果有多个线程同事访问一个临界资源，宁可牺牲性能让线程等待，也不会让一个线程不加锁访问临界资源。对于无锁，它会假定对资源的访问是没有冲突的。也就是多个线程对临界资源的访问是没有冲突的。既然没有冲突，那么就不需要等待，所有的线程都可以不需要等待的执行下去。如果遇到了冲突，怎么办？这里无锁策略使用了一种称为CAS的技术来保证线程执行的安全性。下面我们来具体讨论一下CAS。</p>
<h2 id="无锁解决冲突的办法cas">无锁解决冲突的办法：CAS</h2>
<p>CAS的全称是Compare And Swap即比较和交换。</p>
<p>CAS算法的过程是这样的：它包含三个参数的变量CAS(V, E, N)。作用如下：</p>
<ul>
<li>V 表示要更新的值。</li>
<li>E 表示预期的值。</li>
<li>N 表示新值。</li>
</ul>
<p><strong>仅当V值等于E值的，才会将V值设置为N值</strong>，如果V值和E值不同，则说明已经有其他线程做了更新，当前线程什么都不做。CVS返回的是当前V的真实值。
CAS是乐观派，总认为自己可以操作成功。当多个线程同时使用CAS来给一个变量设置时，只有一个会成功，其它的都会失败，但是CAS很乐观，失败了就失败了，可以再次尝试。</p>
<h2 id="java中的指针unsafe类">Java中的指针：Unsafe类</h2>
<p>What? Java中也有指针。Unsfae类就像它的名字一样，不安全，里面有一些向C语言的指针一样直接操作内存的方法。并且官方也不推荐直接使用Unsafe类。但是CAS的实现用到了这个类。
下面来看看Unsafe中的一些方法：</p>
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<pre tabindex="0" class="chroma"><code class="language-java" data-lang="java"><span class="line"><span class="cl"><span class="c1">// 分配内存
</span></span></span><span class="line"><span class="cl"><span class="c1"></span><span class="kd">public</span> <span class="kd">native</span> <span class="kt">long</span> <span class="nf">allocateMemory</span><span class="o">(</span><span class="kt">long</span> <span class="n">var1</span><span class="o">);</span>
</span></span><span class="line"><span class="cl">
</span></span><span class="line"><span class="cl"><span class="c1">// 重新分配内存
</span></span></span><span class="line"><span class="cl"><span class="c1"></span><span class="kd">public</span> <span class="kd">native</span> <span class="kt">long</span> <span class="nf">reallocateMemory</span><span class="o">(</span><span class="kt">long</span> <span class="n">var1</span><span class="o">,</span> <span class="kt">long</span> <span class="n">var3</span><span class="o">);</span>
</span></span><span class="line"><span class="cl">
</span></span><span class="line"><span class="cl"><span class="c1">// 拷贝内存
</span></span></span><span class="line"><span class="cl"><span class="c1"></span><span class="kd">public</span> <span class="kd">native</span> <span class="kt">void</span> <span class="nf">copyMemory</span><span class="o">(</span><span class="n">Object</span> <span class="n">var1</span><span class="o">,</span> <span class="kt">long</span> <span class="n">var2</span><span class="o">,</span> <span class="n">Object</span> <span class="n">var4</span><span class="o">,</span> <span class="kt">long</span> <span class="n">var5</span><span class="o">,</span> <span class="kt">long</span> <span class="n">var7</span><span class="o">);</span>
</span></span><span class="line"><span class="cl">
</span></span><span class="line"><span class="cl"><span class="c1">// 释放内存
</span></span></span><span class="line"><span class="cl"><span class="c1"></span><span class="kd">public</span> <span class="kd">native</span> <span class="kt">void</span> <span class="nf">freeMemory</span><span class="o">(</span><span class="kt">long</span> <span class="n">var1</span><span class="o">);</span>
</span></span><span class="line"><span class="cl">
</span></span><span class="line"><span class="cl"><span class="c1">// 获取起始地址
</span></span></span><span class="line"><span class="cl"><span class="c1"></span><span class="kd">public</span> <span class="kd">native</span> <span class="kt">long</span> <span class="nf">getAddress</span><span class="o">(</span><span class="kt">long</span> <span class="n">var1</span><span class="o">);</span>
</span></span><span class="line"><span class="cl">
</span></span><span class="line"><span class="cl">
</span></span><span class="line"><span class="cl"><span class="c1">// 获取操作系统内存页大小
</span></span></span><span class="line"><span class="cl"><span class="c1"></span><span class="kd">public</span> <span class="kd">native</span> <span class="kt">int</span> <span class="nf">pageSize</span><span class="o">();</span>
</span></span><span class="line"><span class="cl">
</span></span><span class="line"><span class="cl"><span class="o">.....</span>
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</div>
</div><p>Unsafe中有两个方法可以将线程挂起和恢复，如下：</p>
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<pre tabindex="0" class="chroma"><code class="language-java" data-lang="java"><span class="line"><span class="cl"><span class="c1">// 线程调用方法，将线程挂起
</span></span></span><span class="line"><span class="cl"><span class="c1"></span><span class="kd">public</span> <span class="kd">native</span> <span class="kt">void</span> <span class="nf">unpark</span><span class="o">(</span><span class="n">Object</span> <span class="n">var1</span><span class="o">);</span>
</span></span><span class="line"><span class="cl">
</span></span><span class="line"><span class="cl"><span class="c1">// 线程恢复
</span></span></span><span class="line"><span class="cl"><span class="c1"></span><span class="kd">public</span> <span class="kd">native</span> <span class="kt">void</span> <span class="nf">park</span><span class="o">(</span><span class="kt">boolean</span> <span class="n">var1</span><span class="o">,</span> <span class="kt">long</span> <span class="n">var2</span><span class="o">);</span>
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</div>
</div><p>Unsafe中的关于CAS的操作：</p>
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<pre tabindex="0" class="chroma"><code class="language-java" data-lang="java"><span class="line"><span class="cl"><span class="kd">public</span> <span class="kd">final</span> <span class="kd">native</span> <span class="kt">boolean</span> <span class="nf">compareAndSwapObject</span><span class="o">(</span><span class="n">Object</span> <span class="n">var1</span><span class="o">,</span> <span class="kt">long</span> <span class="n">var2</span><span class="o">,</span> <span class="n">Object</span> <span class="n">var4</span><span class="o">,</span> <span class="n">Object</span> <span class="n">var5</span><span class="o">);</span>
</span></span><span class="line"><span class="cl">
</span></span><span class="line"><span class="cl"><span class="kd">public</span> <span class="kd">final</span> <span class="kd">native</span> <span class="kt">boolean</span> <span class="nf">compareAndSwapInt</span><span class="o">(</span><span class="n">Object</span> <span class="n">var1</span><span class="o">,</span> <span class="kt">long</span> <span class="n">var2</span><span class="o">,</span> <span class="kt">int</span> <span class="n">var4</span><span class="o">,</span> <span class="kt">int</span> <span class="n">var5</span><span class="o">);</span>
</span></span><span class="line"><span class="cl">
</span></span><span class="line"><span class="cl"><span class="kd">public</span> <span class="kd">final</span> <span class="kd">native</span> <span class="kt">boolean</span> <span class="nf">compareAndSwapLong</span><span class="o">(</span><span class="n">Object</span> <span class="n">var1</span><span class="o">,</span> <span class="kt">long</span> <span class="n">var2</span><span class="o">,</span> <span class="kt">long</span> <span class="n">var4</span><span class="o">,</span> <span class="kt">long</span> <span class="n">var6</span><span class="o">);</span>
</span></span></code></pre></td></tr></table>
</div>
</div><p>CAS的操作的核心实现是这三个方法。
三个方法的参数都类似，分别为：</p>
<ol>
<li>CAS需要更改变量的对象；</li>
<li>对象内存的偏移量;</li>
<li>期望值</li>
<li>需要设置的值</li>
</ol>
<p>其中偏移量可以通过Unsafe类中的objectFieldOffset()方法获取到，这些方法如下：</p>
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<pre tabindex="0" class="chroma"><code class="language-java" data-lang="java"><span class="line"><span class="cl"><span class="kd">public</span> <span class="kd">native</span> <span class="kt">long</span> <span class="nf">objectFieldOffset</span><span class="o">(</span><span class="n">Field</span> <span class="n">field</span><span class="o">);</span>
</span></span></code></pre></td></tr></table>
</div>
</div><p>因为int，long，boolean类型的相关操作不是原子性的，所以JDK在1.5之后提供了atomic包（具体在<code>java.util.concurrent.atomic</code>中）来将这些操作变成原子操作。</p>
<p>下面的图片中的是atomic下提供的原子操作的类：
<img
        class="lazyload"
        src="/svg/loading.min.svg"
        data-src="https://img-blog.csdn.net/20181017155907620?watermark/2/text/aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3N0YXJleHBsb2Rl/font/5a6L5L2T/fontsize/400/fill/I0JBQkFCMA==/dissolve/70"
        data-srcset="https://img-blog.csdn.net/20181017155907620?watermark/2/text/aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3N0YXJleHBsb2Rl/font/5a6L5L2T/fontsize/400/fill/I0JBQkFCMA==/dissolve/70, https://img-blog.csdn.net/20181017155907620?watermark/2/text/aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3N0YXJleHBsb2Rl/font/5a6L5L2T/fontsize/400/fill/I0JBQkFCMA==/dissolve/70 1.5x, https://img-blog.csdn.net/20181017155907620?watermark/2/text/aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3N0YXJleHBsb2Rl/font/5a6L5L2T/fontsize/400/fill/I0JBQkFCMA==/dissolve/70 2x"
        data-sizes="auto"
        alt="https://img-blog.csdn.net/20181017155907620?watermark/2/text/aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3N0YXJleHBsb2Rl/font/5a6L5L2T/fontsize/400/fill/I0JBQkFCMA==/dissolve/70"
        title="image.png" /></p>
<p>这里以AtomicInteger来进行分析：</p>
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<td class="lntd">
<pre tabindex="0" class="chroma"><code class="language-fallback" data-lang="fallback"><span class="line"><span class="cl">public class AtomicInteger extends Number implements java.io.Serializable {
</span></span><span class="line"><span class="cl">    
</span></span><span class="line"><span class="cl">    // 实例化Unsafe
</span></span><span class="line"><span class="cl">    private static final Unsafe unsafe = Unsafe.getUnsafe();
</span></span><span class="line"><span class="cl">    
</span></span><span class="line"><span class="cl">    // valueOffset变量保存的是value中的偏移量，这一点可以在下面的static初始化块中可以看出
</span></span><span class="line"><span class="cl">    private static final long valueOffset;
</span></span><span class="line"><span class="cl">
</span></span><span class="line"><span class="cl">    static {
</span></span><span class="line"><span class="cl">        try {
</span></span><span class="line"><span class="cl">            // 获取value的偏移量
</span></span><span class="line"><span class="cl">            valueOffset = unsafe.objectFieldOffset
</span></span><span class="line"><span class="cl">                (AtomicInteger.class.getDeclaredField(&#34;value&#34;));
</span></span><span class="line"><span class="cl">        } catch (Exception ex) { throw new Error(ex); }
</span></span><span class="line"><span class="cl">    }
</span></span><span class="line"><span class="cl">
</span></span><span class="line"><span class="cl">    private volatile int value;
</span></span><span class="line"><span class="cl">
</span></span><span class="line"><span class="cl">    /**
</span></span><span class="line"><span class="cl">     * 通过int值实例化一个AtomicInteger对象
</span></span><span class="line"><span class="cl">     */
</span></span><span class="line"><span class="cl">    public AtomicInteger(int initialValue) {
</span></span><span class="line"><span class="cl">        value = initialValue;
</span></span><span class="line"><span class="cl">    }
</span></span><span class="line"><span class="cl">
</span></span><span class="line"><span class="cl">    /**
</span></span><span class="line"><span class="cl">     * 实例化一个AtomicInteger对象，其初始值为0
</span></span><span class="line"><span class="cl">     */
</span></span><span class="line"><span class="cl">    public AtomicInteger() {
</span></span><span class="line"><span class="cl">    }
</span></span><span class="line"><span class="cl">    
</span></span><span class="line"><span class="cl">    // 获取当前值
</span></span><span class="line"><span class="cl">    public final int get() {
</span></span><span class="line"><span class="cl">        return value;
</span></span><span class="line"><span class="cl">    }
</span></span><span class="line"><span class="cl">    // 设置当前值
</span></span><span class="line"><span class="cl">    public final void set(int newValue) {
</span></span><span class="line"><span class="cl">        value = newValue;
</span></span><span class="line"><span class="cl">    }
</span></span><span class="line"><span class="cl">
</span></span><span class="line"><span class="cl">    // 延迟设值
</span></span><span class="line"><span class="cl">    public final void lazySet(int newValue) {
</span></span><span class="line"><span class="cl">        unsafe.putOrderedInt(this, valueOffset, newValue);
</span></span><span class="line"><span class="cl">    }
</span></span><span class="line"><span class="cl">
</span></span><span class="line"><span class="cl">    // 设置新值获取旧值
</span></span><span class="line"><span class="cl">    public final int getAndSet(int newValue) {
</span></span><span class="line"><span class="cl">        return unsafe.getAndSetInt(this, valueOffset, newValue);
</span></span><span class="line"><span class="cl">    }
</span></span><span class="line"><span class="cl">
</span></span><span class="line"><span class="cl">    // CAS操作，把当前值和预期值做比较，相当时设置新值
</span></span><span class="line"><span class="cl">    public final boolean compareAndSet(int expect, int update) {
</span></span><span class="line"><span class="cl">        return unsafe.compareAndSwapInt(this, valueOffset, expect, update);
</span></span><span class="line"><span class="cl">    }
</span></span><span class="line"><span class="cl">
</span></span><span class="line"><span class="cl">    
</span></span><span class="line"><span class="cl">    public final boolean weakCompareAndSet(int expect, int update) {
</span></span><span class="line"><span class="cl">        return unsafe.compareAndSwapInt(this, valueOffset, expect, update);
</span></span><span class="line"><span class="cl">    }
</span></span><span class="line"><span class="cl">
</span></span><span class="line"><span class="cl">    // 当前值加1，返回旧值
</span></span><span class="line"><span class="cl">    public final int getAndIncrement() {
</span></span><span class="line"><span class="cl">        return unsafe.getAndAddInt(this, valueOffset, 1);
</span></span><span class="line"><span class="cl">    }
</span></span><span class="line"><span class="cl">
</span></span><span class="line"><span class="cl">    // 当前值-1，返回旧值
</span></span><span class="line"><span class="cl">    public final int getAndDecrement() {
</span></span><span class="line"><span class="cl">        return unsafe.getAndAddInt(this, valueOffset, -1);
</span></span><span class="line"><span class="cl">    }
</span></span><span class="line"><span class="cl">
</span></span><span class="line"><span class="cl">    // 当前值加上delta，然后返回旧值
</span></span><span class="line"><span class="cl">    public final int getAndAdd(int delta) {
</span></span><span class="line"><span class="cl">        return unsafe.getAndAddInt(this, valueOffset, delta);
</span></span><span class="line"><span class="cl">    }
</span></span><span class="line"><span class="cl">
</span></span><span class="line"><span class="cl">    // 当前值加1，然后返回
</span></span><span class="line"><span class="cl">    public final int incrementAndGet() {
</span></span><span class="line"><span class="cl">        return unsafe.getAndAddInt(this, valueOffset, 1) + 1;
</span></span><span class="line"><span class="cl">    }
</span></span><span class="line"><span class="cl">    
</span></span><span class="line"><span class="cl">    // 当前值减1，返回返回
</span></span><span class="line"><span class="cl">    public final int decrementAndGet() {
</span></span><span class="line"><span class="cl">        return unsafe.getAndAddInt(this, valueOffset, -1) - 1;
</span></span><span class="line"><span class="cl">    }
</span></span><span class="line"><span class="cl">
</span></span><span class="line"><span class="cl">    // 当前值加上delta，然后返回
</span></span><span class="line"><span class="cl">    public final int addAndGet(int delta) {
</span></span><span class="line"><span class="cl">        return unsafe.getAndAddInt(this, valueOffset, delta) + delta;
</span></span><span class="line"><span class="cl">    }
</span></span><span class="line"><span class="cl">
</span></span><span class="line"><span class="cl">    // ...
</span></span><span class="line"><span class="cl">}
</span></span></code></pre></td></tr></table>
</div>
</div><p>其中的大部分方法都直接或者间接使用了CAS来保证安全。</p>
<p>除了上面的对于基本变量的Atomic类，还有关于普通对象引用的Atmoic类。</p>
<h2 id="atomicreference-无锁的引用">AtomicReference 无锁的引用</h2>
<p>AtomicReference和AtmoicInteger非常类似，不同之处就是AtmoicInteger是对整数的封装，而AtmoicReference是对普通对象的引用，也就是它可以保证在修改对象引用时线程的安全性。</p>
<p>CAS中有一个很重要的问题ABA。CAS比较的是对象中的值和期望值，但是有可能在你获取到当前对象的数据后，在准备修改为新值之前，对象的值被其他线程连续修改两次，而且经过这两次修改之后，对象有恢复为旧值。这样，前后的结果看似没有被改过，但是其实已经被修改了2次。过程如下图：
<img
        class="lazyload"
        src="/svg/loading.min.svg"
        data-src="https://img-blog.csdn.net/20181010171642689?watermark/2/text/aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3N0YXJleHBsb2Rl/font/5a6L5L2T/fontsize/400/fill/I0JBQkFCMA==/dissolve/70"
        data-srcset="https://img-blog.csdn.net/20181010171642689?watermark/2/text/aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3N0YXJleHBsb2Rl/font/5a6L5L2T/fontsize/400/fill/I0JBQkFCMA==/dissolve/70, https://img-blog.csdn.net/20181010171642689?watermark/2/text/aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3N0YXJleHBsb2Rl/font/5a6L5L2T/fontsize/400/fill/I0JBQkFCMA==/dissolve/70 1.5x, https://img-blog.csdn.net/20181010171642689?watermark/2/text/aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3N0YXJleHBsb2Rl/font/5a6L5L2T/fontsize/400/fill/I0JBQkFCMA==/dissolve/70 2x"
        data-sizes="auto"
        alt="https://img-blog.csdn.net/20181010171642689?watermark/2/text/aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3N0YXJleHBsb2Rl/font/5a6L5L2T/fontsize/400/fill/I0JBQkFCMA==/dissolve/70"
        title="在这里插入图片描述" /></p>
<p>一般来说，发生这种事情的可能性很小。而且即使发生了也不会有什么影响，比如，一个数字，被修改一次后，在修改回去，不会对CAS产生什么影响。</p>
<p>但是有时候在一些具体问题中这种情况就有可能发生。所以在JDK中提供了AtomicStampedReference来解决这种问题。</p>
<h2 id="atomicstampedreference">AtomicStampedReference</h2>
<p>AtomicStampedReference在内部维护了一个时间戳。当AtmoicStampedReference对应的数字被修改时，除了更新数据本身，还必须更新时间戳。当AtomicStampedReference设置对象值时，对象和时间戳都必须满足期望值，才会写入成功。通过维护时间戳能有效的防止ABA问题。</p>
<p>AtmoicStampedReference的几个API在Atmomic的基础上新增了几个有关时间戳的信息。</p>
<div class="highlight"><div class="chroma">
<table class="lntable"><tr><td class="lntd">
<pre tabindex="0" class="chroma"><code><span class="lnt"> 1
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</span></code></pre></td>
<td class="lntd">
<pre tabindex="0" class="chroma"><code class="language-java" data-lang="java"><span class="line"><span class="cl"><span class="c1">// 参数为：期望值，新值，期望时间戳，新的时间戳
</span></span></span><span class="line"><span class="cl"><span class="c1"></span><span class="kd">public</span> <span class="kt">boolean</span> <span class="nf">compareAndSet</span><span class="o">(</span><span class="n">V</span>   <span class="n">expectedReference</span><span class="o">,</span>
</span></span><span class="line"><span class="cl">                                 <span class="n">V</span>   <span class="n">newReference</span><span class="o">,</span>
</span></span><span class="line"><span class="cl">                                 <span class="kt">int</span> <span class="n">expectedStamp</span><span class="o">,</span>
</span></span><span class="line"><span class="cl">                                 <span class="kt">int</span> <span class="n">newStamp</span><span class="o">);</span>
</span></span><span class="line"><span class="cl"><span class="c1">// 获取索引
</span></span></span><span class="line"><span class="cl"><span class="c1"></span><span class="kd">public</span> <span class="n">V</span> <span class="nf">getReference</span><span class="o">()</span>
</span></span><span class="line"><span class="cl"><span class="c1">// 获取时间戳
</span></span></span><span class="line"><span class="cl"><span class="c1"></span><span class="kd">public</span> <span class="kt">int</span> <span class="nf">getStamp</span><span class="o">()</span>
</span></span><span class="line"><span class="cl"><span class="c1">// 设置当前对象引用和时间戳
</span></span></span><span class="line"><span class="cl"><span class="c1"></span><span class="kd">public</span> <span class="kt">void</span> <span class="nf">set</span><span class="o">(</span><span class="n">V</span> <span class="n">newReference</span><span class="o">,</span> <span class="kt">int</span> <span class="n">newStamp</span><span class="o">)</span>
</span></span></code></pre></td></tr></table>
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